How rainwater harvesting is protecting borehole supplies

Rainwater captured from their floating slurry lagoon cover is probably saving about £10,000 a year in tankering costs for Rheinallt and Rachel Harries at Llwynmendy Uchaf, Bethlehem, Carmarthenshire.
Instead of 3.4m litres of rainwater diluting slurry in the 9,693cu m lagoon, it is pumped off, filtered to remove debris, UV-treated and used to wash down the parlour and collecting yard.
“We have a 7,000-litre volume washer tank and we fill it daily using 10,000 litres of rainwater after our two milkings,” says Rheinallt.
See also: Harvesting rainwater: The benefits and how to do it on your farm
“Although we are on a borehole, as cow numbers have gone up, so has water demand, and we don’t want to deplete it, especially in a dry summer. Water is a valuable commodity, and we have to look after what we’ve got.”
The lagoon was installed after rising livestock numbers, plus a few more uncovered yards, outgrew the farm’s original 1.1m litre slurry store.
“It was a nuisance as we were always looking at the forecast for a couple of dry days to get slurry out,” he recalls.
Farm facts: Llwynmendy Uchaf, Bethlehem, Carmarthenshire
- Tenant on 197ha, including 60ha grazing platform
- Own 11ha smallholding with housing for 95 heifers plus a holiday cottage
- 200 crossbred cows
- 134 replacements
- Spring block-calving
- 5,400 litres at 4.76% fat and 3.74% protein
- Beef calves sold privately
- Five-year average rainfall 1,700-2,100mm
Investment in storage
Pre-empting Nitrate Vulnerable Zone requirements, the couple decided five years ago to invest in 12 months’ storage.
This allows them to apply slurry strategically when needed; using it as a fertiliser has halved their bought-in nitrogen to 124kg/ha.
A lagoon with internal banks of 64x39m and 6m deep was sited 80m from the farm buildings and away from the nearby River Towy.
“We got a £20,000 grant on the excavations, and £8,000 towards a floating cover, which cost £20,000,” says Rheinallt.
“I’d read they would be compulsory in England by 2027 and thought they might come our way eventually.
“The downside of a bigger footprint is that you catch a lot of rain.
“At the time, it would have cost £8,000 a year to carry this back out of the lagoon – or we would have more product to deal with – and prices have gone up since then,” he explains, adding that it was more cost-effective in the long run to cover it.

Rachel and Rheinallt Harries © Richard Stanton
Floating cover
Rheinallt opted for a rigid, high-density polyethylene cover in 4m rolls that were glued together across the store surface.
“The cover floats on top and when the lagoon is empty it sits on top of the base liner. As the slurry level fills, it brings the top cover up with it. It has a 20-year guarantee on it.”
However, only 30cm depth of rainwater can be left on the cover to weigh it down and stop the wind from lifting it up. Any more than this affects the slurry bubbler’s ability to work, he says.
To drain the excess, Rheinallt set up an old borehole pump in a repurposed chemicals barrel attached to 32mm piping, to take water down to the volume washer tank at the dairy.
“With hindsight, it should have been 50mm piping – we are now on our second pump as we can get some leaves in and it’s pumping every day.
“We might put a small header tank at the lagoon and filter the water, then have a pontoon to keep the barrel afloat and pull in less leaves and rubbish.”
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